Thanks for the replies everybody. As I was trying to loosen the battery cable lead from the starter solenoid the lead stud broke off and the solenoid housing broke in half along with it. No visible corrosion on the leads other than normal dust/crud.
I went ahead and replaced the starter motor and voila, problem solved! I think the solenoid was the culprit (rusty/dirty inside) and guess I could have replaced only that but the starter motor was cheap enough figured might as well do the whole thing. And yes, Tech II, there was a heat shield installed which I transferred to the new starter.